Certain extremely obese individuals are believed to have decreased life expectancy associated with a high incidence of abnormal systemic conditions such as hypertension, diabetes, hyperlipidemia, and pulmonary dysfunction meriting their condition the descriptive diagnosis of morbid obesity. The utilization of metabolic surgery such as small intestinal bypass to produce an obligatory weight loss has resulted in the reversal of the pathologic systemic processes associated with the obese state. Unfortunately, bypass of large amounts of small bowel is associated with many physiologic alterations (complications) which themselves decrease function and produce morbidity. The most important of these complications is hepatic fat accumulation, primarily because it produces the greatest mortality. In an attempt to determine the cause of hepatic lipid accumulation following small bowel bypass an obese rat model was produced and evaluated by the principal investigator which allows the ready evaluation of the effects of the operation on hepatic metabolism. Studies performed in this animal model have resulted in relevant information relating to post-bypass steatosis. A non-obese rat, morphologically, has relatively little accumulation of fat in the liver post-bypass compared to a nutritionally obese rat. Hepatic lipid accumulation, quantitatively determined at various intervals post-bypass in a nutritionally obese rat is greatest during the period of accelerated weight loss and plateaus with decreasing weight loss. This phenomenon also occurs post-bypass in humans and suggests important adaptive changes are occurring. Subsequent studies have demonstrated that the increased hepatic lipid accumulation produced by choline deficiency is exacerbated by small bowel bypass. This is most likely the result of protein deficiency removing the possible use of the methionine supplied methyl group as a dietary choline substitute. Continued performance of metabolic and absorptive studies in an obese rat model may provide insight into the mechanisms producing many metabolic abnormalities that occur in morbidly obese humans undergoing small intestinal bypass.